Wow Doc, what a plot twist.
I really don't know what to say to make you feel better. You should definitely reply to Elsa though.
Just say "hey" back and see where it goes....
Like I said in your OP, good friendships are important, worth trying to salvage. Best not to hesitate if she's leaving the door ajar, as it may be pushed closed suddenly...
I would also continue to avoid Christof, doesn't matter what his story is it comes down to him either being spineless or venomous, neither being desirable traits.